The invention relates to a device for measuring the pulse transfer spectrum of X-ray quanta which are elastically scattered in an examination zone, comprising a polychromatic X-ray source which is arranged on one side of the examination zone and a detector device which is situated on an other side of the examination zone and which measures the energy of the X-ray quanta, and also comprising two diaphragm devices which have a common symmetry axis and transmit scattered radiation to the detector only within a given scattering angle range, the first diaphragm device being arranged between the examination zone and the X-ray source whilst the second diaphragm device is arranged between the examination zone and the detector device. A device of this kind is essentially known from German patent application P 37 12 928.
It is known that scattered radiation which encloses only a small angle with respect to the primary radiation (for example, an angle of less than 10.degree.) consists mainly of elastically scattered radiation if the energy of the X-ray quanta is not very high. Contrary to non-elastically scattered radiation (Compton scattered radiation), the energy spectrum of elastically scattered radiation corresponds to that of the primary radiation beam. The intensity of elastically scattered radiation exhibits a strong dependency on the pulse transfer which is determined by the molecular structure of the irradiated substance.
This dependency, also being different for fat and muscle tissue, is utilized by the known device in order to form a fat image and a muscle tissue image, respectively. The scattered radiation produced by a monochromatic radiation source is then measured for the pulse transfer range for which the spectrum of the relevant tissue (fat or muscle) is as large as possible whilst that for the other type of tissue (muscle or fat) is as small as possible.
The cited Application P 37 12 928 also mentions that instead of a monochromatic source there may also be provided a polychromatic gamma source or X-ray source if use is made of energyresolution detectors. The pulse transfer spectrum can thus be determined.
From such a spectrum information concerning the molecular structure of the irradiated substance, can be derived in order to identify this substance. This is useful for medical applications, for example for determining the mineral contents of bones in diagnosing osteoporosis, but also for other purposes, for example the inspection of luggage.
The pulse transfer of an X-ray quantum is at least approximately proportional to the product of the energy of the X-ray quantum and the scattering angle at which the X-ray quantum has been deflected from its previous path during the scattering process. The accuracy of determination of the pulse transfer depends inter alia on the accuracy of determination of the energy of the X-ray quantum and its scattering angle. The energy of X-ray quanta can be quite accurately measured by means of a suitable detector, for example made of germanium. In the described device the accuracy of determination of the scattering angle depends on the two diaphragm devices. The first diaphragm device is constructed so that a beam of small cross-section (pencil beam) is formed. The second diaphragm device consists of a number of laminations which are arranged on the envelopes of cones whose apices are situated on the primary beam formed in the examination zone. If the scattering angle is to be defined as accurately as possible in this manner, the lamination dimensions must be as large as possible in comparison with the thickness of the examination zone. In the case of thick objects, this implies the use of very long laminations and hence detectors having a very large diameter. Such detectors are expensive.